In the prior art, it has in the past been proposed to provide various kinds of power assistance means for assisting the rotation of the steering column in a steerable vehicle. In general, such a mechanism includes a motorised speed reducing means comprising essentially an electric motor and a speed reducing means or gearbox. The output shaft of the motor is coupled through an output pinion, or any other suitable means of transmission, in such a way that it is coupled mechanically with the steering column or with a pinion or gear wheel mounted on the latter. Because of this, detection of the forces exerted by the driver on the steering wheel, for example by means of a sensor for detecting angle of rotation or a sensor which detects the torque exerted on the steering wheel, enables a command to be produced for appropriate proportional action of the motorised speed reducing means, so as to apply the required additional power to assist operation of the steering column.
Pneumatic or hydraulic power assistance systems for steering have also been proposed, although the preferred embodiment of the present invention uses an electric motor as the source of mechanical energy to be used for assisting the steering. In a first type of power assisted steering system, the motorised speed reducing means or source of mechanical assistance energy is compelled to follow complex instructions such that it can provide power assistance under widely differing driving conditions. Such systems are expensive and complicated, and their reliability is affected accordingly.
Therefore, it has also been proposed to arrange the power assisted steering system such that it is only activated temporarily, i.e. during those periods when assistance is actually required, for example when it is called for by particular driving conditions or when the driver is exerting too much effort on the steering wheel. Such a system can therefore be said to be acting intermittently. Power assistance is especially valuable during parking, while the vehicle is moving slowly and the driver has a frequent need to exert relatively high forces on the steering wheel. However the same is also true for steering at low speed in general.
In this type of power assisted steering system, when the speed of the vehicle exceeds a certain limited value the power assistance is no longer operative, and the driver feels a resistive torque on the steering column. This resistive torque is caused partly by the forces exerted by the road on the road wheels, and partly by the inertia of the motorised speed reducing means, that is to say mainly the inertia of the rotatable parts of the latter. This feature of systems in the prior art is of special disadvantage in that the multiplication ratio of the speed reducer causes the inertia of the rotor of the electric motor to play a dominant part in the generation of the resistive forces. This somewhat negates the additional convenience afforded by this kind of temporary power assistance mechanism, even though it does have the advantage of being cheaper than a conventional "all speeds" power assistance system.